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THE DECISION BETWEEN WHICH antifriction bearing type to specify on National Electrical Manufacturers Associa- tion (NEMA) motors is not always easy or able for heavy belted loads (i.e., roller bearing), and the motor is directly connected, the bearing may prematurely fail due to a lack of maintaining the required minimum radial load. Often, less than optimal bearing selection can

obvious. From an end user’s perspective, it would be sim- plest if each motor would only have one bearing configu- ration available and have that bearing configuration be suitable for both direct-connected and belted loads. Unfor- tunately, that is currently not the case. This is particularly true on the larger higher-speed motors (larger than 125 hp and faster than 1,200 r/min). If a bearing is selected that is optimized for direct connection (i.e., deep grooveball bearing), and it is belted, the bear- ing may prematurely fail due to a mechanical overload condition. On the other

hand, if a bearing is selected that is suit-

work if special attention is given towards cleanliness, relu- brication, alignments, etc. This article discusses the rela- tionships between these various factors and bearing selection and helps the reader understand the tradeoffs involved in the various applications of antifriction bear- ings and suggests alternate solutions.68692

History

Within the pulp and paper industry, an electric motor may be connected to the driven load by two methods: coupled and belted. Some variations in this exist, such as inserting a quill shaft into a gearbox or using a vertical pump motor that requires special analysis of radial and axial loads; these will not be addressed in this article.

A coupled load is where the motor shaft is usually connected by a flexible coupling to the driven load. This type of load presents no axial or radial load to the motor bearings except for the weight of the motor’s rotor and shaft assembly (misalignment from mounting errors can, however, add radial load). Antifriction (ball) or hydrodynamic (sleeve) bearings are commonly used suc- cessfully for a coupled load.

The most common type of belted load is when a V- groove pulley is mounted on the motor shaft and is con- nected to another pulley on the driven load by means of one or more belts held in tension. This type of load can generate high radial loads on the motor shaft, mostly at the drive-end bearing because it is closest to the applied external loading. Depending on the magnitude of radial load, antifriction bearings (either ball or roller) are used for belted loads.

On smaller motors (through about 100 hp in NEMA 404-5T frames), under normal conditions, a ball bear- ing may be used for either coupled or belted loads (many times bearing L10 life is the key—typically a minimum of 50,000 h for belted loads). If the mini- mum load requirements are met for the rolling bear- ings, then the bearing life is often limited by the grease lubrication life. As motors get larger than 100 hp, ball bearings may still be used on coupled loads, but belted loads should be evaluated for load on the bearing shaft and a roller bearing with higher load capacity and pos- sible misalignment capability selected, if required. Roller bearings, however, require higher minimum loading for operation than do ball bearings.

Motor Construction

A requirement for two types of motors for different load- ing conditions often causes problems in mills. If a 100+ hp motor with ball bearings is used on a heavy belted load, the drive end bearing could fail after a short time from overloading. If a motor designed for heavy belting is used on a coupled load, there may not be enough radial loading to cause the rolling elements to roll. They will

skate or skid on the race, causing high temperatures and sometimes rapid and traumatic failure. Underloading of a roller bearing, even with a belted load, can result in premature failure from the same skidding phenomena.